1. Field of the Invention
The invention relates to a device for stiffening a large, thin workpiece having low inherent stiffness. More particularly, the invention relates to a fixture for holding such a workpiece during a machining operation.
2. Discussion of the Prior Art
When machining large, thin aluminum sheets, for example, it is desirous to reduce the weight of the workpiece and this is commonly done by machining, i.e., milling, away material, so as to reduce the layer thickness in certain areas. This is commonly done in the field of aircraft construction. It is, of course, important to maintain high machining accuracy and avoid undesirable degradation of the components resulting from removing too much material. To this end, it is necessary to guide the machining tool, e.g., a cutter head, as precisely as possible, whereby the precise position of the workpiece also has to be maintained. Any flexibility in the workpiece can cause it to yield under the machining pressure of the tool, with the result that the workpiece is not machined according to specification. To avoid such yielding by the workpiece, clamping means are used that clamp and stiffen such workpieces.
It is known in the field to use so-called clamping tables that are equipped with a large number of actuators arranged in a matrix. The actuators generally have suction devices on their free end, which serve to hold and fix the workpiece in position. The actuators are individually adjustable, that is, the height and angle of the suction device of each actuator is adjustable independently of other actuators, so that an entire array of actuators can be adapted to correspond to the specified contour of the workpiece. Each individual actuator can cost more then 1,000.00 Euros and, because the actuators should be placed as close together as possible to ensure optimum support of the workpiece, such clamping tables are relatively expensive. In practice, however, it is common to see thin-walled workpieces buckling in the spaces between the actuators, that is, yielding under the machining tool. These areas are then insufficiently machined. Machining to specification the requires that the workpiece be re-worked, and this re-work machining requires a great deal of effort regarding fixtures and devices and measurement techniques.